Structure and method for electrical isolation of optoelectronic integrated circuits

ABSTRACT

Semiconductor devices in an optoelectronic integrated circuit are electrically isolated from each other by using planar lateral oxidation to oxidize a buried semiconductor layer vertically separating the semiconductor devices.

BACKGROUND OF INVENTION

In a variety of semiconductor applications it is desirable to accessburied semiconductor layers in order to oxidize the buried layer.Numerous photonic, electronic and micro-mechanical devices require aregion of buried material that is electrically insulating or differsfrom surrounding material by having a lower refractive index. Such amaterial can be formed during device fabrication by selectivelyconverting one or more buried semiconductor layers into an electricallyinsulating low refractive index native oxide.

An oxidized region may be made to partially or completely surround aregion of interest to produce a desired optical effect due to the lowerrefractive index of the oxide region. Buried oxide layers may alsofunction to electrically isolate different regions of a device or toelectrically isolate one device from another on the same wafer. Theburied layer material may be any material that oxidizes rapidly in alateral direction and is typically a semiconductor having a highaluminum content such as AlGaAs, AlGaInP or AlAsSb. The buried layer maybe, but is not limited to compounds containing aluminum and one or moreof the following elements: As, Ga, In, P and Sb. Aluminum will typicallycomprise at least 70% of the Group III component of the compound.Exposure to an oxidizing environment such as steam at elevatedtemperature the buried layer would oxidize laterally, proceeding fromexposed sidewalls inward towards unoxidized portions of the material.The lateral oxidation rate generally increases with increasing aluminumcontent.

The usual method of accessing the buried oxidation layers is through amesa etch. This method leads to a high level of wafer non-planarity thatcomplicates subsequent processing steps. Moreover, the large amount ofmaterials removed degrade the device's mechanical integrity andincreases its thermal resistance. The problem is especially severe indevices like vertical-cavity surface-emitting lasers, where theoxidation layer is usually embedded far beneath the wafer surface.

SUMMARY OF INVENTION

Instead of forming mesas that expose the sidewalls of the layers to beoxidized, a plurality of etched cavities may be used to access theburied layer for oxidation. The shape and size of the resulting oxidizedregion are defined by the shape of each cavity and by placement of thecavities with respect to each other. The area between cavities remainsplanar, which eases further processing steps such as electrical contactformation and photolithography. Planar structures allow for simple etch,deposition, photolithography steps without concern for depth of focusissues during photolithography or problems with step coverage duringdeposition or spin coating of dielectric and or polymer films. Thematerials between etched cavities remain intact, so good mechanicalintegrity and thermal conductivity can be maintained.

Planar lateral oxidation benefits a number of applications. Applicationsinclude but are not limited to applications containing a core regionthat is surrounded by buried oxidized materials, where the oxidizedmaterials provide optical waveguiding, a defined electrical conductionpath, or both; applications where one or more completely oxidized layersare used for their optical filtering properties; applications thatemploy the oxidized layers for electrical isolation; and applicationswhere properties are controlled by the shape of the oxidized region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows a top view of an embodiment in accordance with theinvention.

FIG. 1b shows a cross-sectional view of the embodiment in FIG. 1a inaccordance with the invention.

FIG. 1c shows variation of the lateral index of refraction step as afunction of oxide layer thickness for an embodiment in accordance withthe invention.

FIG. 1d shows the maximum width of the stripe needed to maintain singlemode operation as a function of the spacing between the laterallyoxidized layer and the separate confinement heterostructure.

FIG. 2 shows a mesa structure edge emitting semiconductor laser.

FIG. 3a shows a top view of an embodiment in accordance with theinvention.

FIG. 3b shows a cross-sectional view of the embodiment in FIG. 3a inaccordance with the invention.

FIG. 4a shows a top view of an embodiment in accordance with theinvention.

FIG. 4b shows a cross-sectional view of the embodiment in FIG. 4a inaccordance with the invention.

FIG. 5 shows a top view of an embodiment in accordance with theinvention.

FIG. 6 shows a cross-sectional view of an embodiment in accordance withthe invention.

FIG. 7a shows a top view of an embodiment in accordance with theinvention.

FIG. 7b shows a cross-sectional view of the embodiment in FIG. 7a inaccordance with the invention.

FIG. 7c shows a cross-sectional view of the embodiment in FIG. 7a inaccordance with the invention.

FIG. 8 shows a cross-sectional view of an embodiment in accordance withthe invention.

FIG. 9 shows a top view of an embodiment in accordance with theinvention.

FIG. 10 shows a cross-sectional view of the embodiment in FIG. 9 inaccordance with the invention.

FIG. 11a shows a top view of an embodiment in accordance with theinvention.

FIG. 11b shows a top view of an embodiment in accordance with theinvention.

FIG. 13a shows a top view of an embodiment in accordance with theinvention.

FIG. 13b shows a cross-sectional view of the embodiment in FIG. 13a inaccordance with the invention.

FIGS. 14a-d show a side view of steps for making an embodiment inaccordance with the invention.

FIGS. 15a-d show a top view of the steps in FIGS. 14a-d, respectively.

DETAILED DESCRIPTION

An embodiment in accordance with the invention, edge emitting laserstructure 100 is shown in FIG. 1a in top view and in FIG. 1b in crosssection. Ga_(0.4)In_(0.6)P quantum well active layer 175, typicallyabout 80 Å thick, is bordered by p-type(Al_(0.6)Ga_(0.4))_(0.5)In_(0.5)P barrier layer 186 and by n-type(Al_(0.6)Ga_(0.4))_(0.5)In_(0.5)P barrier layer 185, both barrier layers185 and 186 typically being about 0.12 μm thick. Etched cavities 110allow access to Al_(y)Ga_(1−y)As buried layer 120 for lateral oxidation.The value selected for y is generally greater than 0.95 and a typicalvalue is 0.99. Al_(0.99)Ga_(0.01)As buried layer 120 is typically lessthan 1000 Å thick. Stripe 170 of Al_(0.99)Ga_(0.01)As buried layer 120remains unoxidized to provide a lateral index of refraction step whileregions 123 are oxidized to define stripe 170. GaInP layer 111 isoptional and lies below Al_(0.99)Ga_(0.01)As buried layer 120 and servesas an etch stop for etching of cavities 110. P type Al_(0.05)In_(0.05)Pcladding layer 155 overlies Al_(0.99)Ga_(0.01)As buried layer 120 whilen type Al_(0.05)In_(0.05)P cladding layer 156 lies below n-type(Al_(0.6)Ga_(0.4))_(0.5)In_(0.5)P barrier layer 185 and above n⁺ GaAssubstrate 171. Both cladding layer 155 and cladding layer 156 aretypically about 1 μm thick. Electrical connections are provided byp-electrode 105 and n-electrode 106. Electrical isolation is achieved byimplanting regions 159 using a shallow proton implant.

Following oxidation, Al_(0.99)Ga_(0.01)As buried layer 120 provideslateral index guiding for edge-emitting laser structure 100.Edge-emitting laser structure 100 is made single mode by tailoring thethickness and position of Al_(0.99)Ga_(0.01)As buried layer 120 tocreate an effective lateral index of refraction step to allow only forpropagation of the fundamental mode. The lateral index of refractionstep is determined by the effective index of refraction of three slabwaveguides. Two slab waveguides are defined by the regions lying aboveand below the oxidized portions of layer 120 while the third waveguideregion is defined by the layers lying above and below unoxidized stripregion 170. The lateral index of refraction step depends on thethickness of Al_(0.99)Ga_(0.01)As buried layer 120 and the distance, x,of Al_(0.99)Ga_(0.01)As buried layer 120 from separate confinementheterostructure (SCH) 190 which consists of layers 186, 175 and 185.Curve 166 in FIG. 1c shows the dependence of the lateral index ofrefraction step as a function of the thickness of Al_(0.99)Ga_(0.01)Asburied layer 120 in angstroms. FIG. 1d shows the relationship betweenthe maximum width of stripe 170 in microns needed to maintain singlemode operation and the distance, x, in angstroms, ofAl_(0.99)Ga_(0.01)As buried layer 120 having a thickness of 500 Å fromSCH 190. Points in FIG. 1d lying below curve 167 indicate single modeoperation while points above curve 167 indicate multi-mode operation ofedge-emitting laser 100.

Because overall wafer planarity is maintained, electrical contacting bymetal contact 105 can be readily accomplished without complicatedplanarization steps. Metal contact 106 at the base of edge-emittinglaser 106 serves as the second electrical contact. The embodiment inaccordance with the invention shown in FIGS. 1a and 1 b differs fromedge-emitting laser 300 shown in FIG. 2 where a mesa etch is used toaccess oxidation layers 120 and 121. The processing steps used tofabricate edge emitting laser 100 in FIGS. 1a and 1 b are similar tothose employed in FIG. 2 except cavities 110, instead of mesa 310, areemployed to allow water vapor to access buried layer 120 and arediscussed below.

The use of laterally oxidized Al_(0.99)Ga_(0.01)As buried layer 120requires minimum etching, only etching of etched cavities 110 to accessAl_(0.99)Ga_(0.01)As buried layer 120. Since Al_(0.99)Ga_(0.01)As buriedlayer 120 is typically less than 1000 Å thick which means that nosignificant changes need be made to already successful laser designs andthere is no significant increase in the time needed for epitaxial growthof the laser structure.

The planar lateral oxidation can be extended to the design of multiplebeam edge-emitting lasers. The planar lateral oxidation used for thedesign of multiple beam edge emitting laser structures in accordancewith the invention allows easy electrical routing to achieve closespacing, typically smaller than 20 μm, between individual edge-emittinglasers 325, 326, 327 and 328 as shown in FIG. 3a. FIG. 3a shows a planarview of quad beam structure 300 in accordance with the invention thatconsists of four closely-spaced independently addressable edge-emittinglasers 325, 326, 327 and 328 while FIG. 3b shows a cross-sectional viewof quad beam embodiment 300 along line 3 b in FIG. 3a. Electricalisolation between edge emitting lasers 325, 326, 327 and 328 isaccomplished through shallow proton implant regions 340 in FIG. 3b.Cavities 310 are present to allow access for performing oxidation ofburied layers 320 and 321. Following oxidation, buried layers 320 and321 provide lateral index guiding for edge-emitting lasers 325, 326, 327and 328. Metal contact 350 contacts laser 326, metal contact 360contacts laser 325, metal contact 351 contacts laser 327 and metalcontact 361 contacts laser 328 with common metal contact 306 contactinglasers 325, 326, 327 and 328. Active region 375 for edge-emitting lasers325, 326, 327 and 328 is located between buried layers 320 and 321.

Passive waveguides can be formed using cavities to allow oxidation ofburied layers. A region that is bordered by a lower refractive indexmaterial is capable of confining light and can be used to guide lightfrom one location to another. Oxidized semiconductors typically haverefractive indices that are lower than unoxidized semiconductors. Forexample, at a typical λ=980 nm, AlAs has a refractive index of about 2.9and GaAs has a refractive index of about 3.5 whereas oxidized AlAs has arefractive index of 1.5. When two waveguides are close enough to each,typically within about 0.15λ or 147 nm for λ=980 nm, the optical mode inone waveguide couples to the other through evanescent wave interaction.

FIG. 4a is a planar view showing passive waveguide structure 400 inaccordance with the invention which functions as a three way beamsplitter. Passive waveguide structure 400 consists of four waveguidescores 425, 426, 427 and 428 (see also FIG. 4b which shows across-sectional view along line 5 of FIG. 4a). Waveguide core 428 isevanescently coupled to waveguide cores 425, 426 and 427 resulting in athree way split of the signal in waveguide core 428.

Typically, passive waveguide structure 400 is made on GaAs substrate450. Al_(0.99)Ga_(0.01)As layer 438 and Al_(0.99)Ga_(0.01)As layer 437are placed above and below GaAs layer 436 to vertically confine light inwaveguide core 428 in GaAs layer 436. Al_(0.99)Ga_(0.01)As layer 438 isgrown on GaAs substrate 450. GaAs layer 455 is grown onAl_(0.99)Ga_(0.01)As layer 437. Al_(0.99)Ga_(0.01)As layer 430 is thengrown over GaAs layer 455 and GaAs capping layer 440 is grown overAl_(0.99) Ga_(0.01)As layer 430. Subsequently, cavities 410 and 411 areetched to provide access to Al_(0.99)Ga_(0.01)As layer 430 andAl_(0.99)Ga_(0.01)As layer 437 for oxidation. Cavities 411 in FIG. 4bare etched down to Al_(0.99)Ga_(0.01)As layer 437 and cavities 410 inFIG. 4b are etched down to Al_(0.99)Ga_(0.01)As layer 430. Side portions422 of cavities 410 and side portions 420 of cavities 411 inAl_(0.99)Ga_(0.01)As layer 430 are laterally oxidized using water vaporto provide transverse confinement for waveguide cores 425, 426 and 427in GaAs layer 455. Side portions 421 of cavities 411 are laterallyoxidized using water vapor to provide transverse confinement forwaveguide core 428. The lateral extent of oxidation for portions 420,421 and 422 is typically about 4 μm for the embodiment shown in FIG. 4bwith etched cavities 411 having a width of about 2 μm.

FIG. 5 shows an embodiment in accordance with the invention which actsas a beam splitter to allow, for example, light to be split off to aphotodetector or spectrophotometer. Light enters waveguide core 526which is defined by cavities 510 with a coupled optical mode beinggenerated by evanescent wave interaction in waveguide core 527 which isdefined by cavities 511. Waveguide cores 526 and 527 are verticallystacked and do not lie in the same horizontal plane. Transverseconfinement is created by lateral oxidation of the Al_(0.99)Ga_(0.01)Aslayers (not shown) over the GaAs layer containing waveguide core 526 andthe GaAs layer containing waveguide core 527, respectively. Cavities 510and 511 are etched down to the respective Al_(0.99)Ga_(0.01)As layers(not shown).

Another category of devices includes those where the oxidized layers areused as an anti-reflection coating, a distributed Bragg reflector (DBR),or as a way to form a microlens. Because the difference in refractiveindex of the oxide and an adjacent semiconductor material can be mademuch bigger than can be achieved if the layer is not oxidized, a DBRmirror can have a very high reflectivity bandwidth that is relativelyinsensitive to variations in layer thickness. Also, fewer mirror layersare necessary to achieve a given reflectivity, reducing crystal growthtime and cost. For example, at a wavelength of 980 nm the refractiveindex difference between GaAs and oxidized AlAs is about 2 which isabout 3.5 times larger than the refractive index difference between GaAsand AlGaAs alloys. The large refractive index difference between GaAsand AlAs allows a high reflectivity with a wide reflective bandwidth.Since the high reflectivity bandwidth is more than five times wider thanfor unoxidized materials the layer thickness tolerances necessary toproduce high reflectivity at a particular wavelength is also about fivetimes more relaxed and the reflection spectrum is relatively insensitiveto variations in layer thickness. Therefore, fewer layers and lessstringent thickness tolerances are needed to obtain a givenreflectivity. Nineteen pairs of GaAs/AlAs layers are typically needed toachieve a reflectivity of about 99.8% compared to four pairs ofGaAs/oxidized AlAs layers.

An embodiment in accordance with the invention is shown in FIG. 6 insimplified cross-section. AlAs layers 620 alternate with GaAs layers 630to form DBR mirror 600. Cavities 610 are used to access AlAs layers 620for oxidation. The shaded portion of AlAs layers 620 indicates theextent of the oxidation in AlAs layers 620.

In accordance with an embodiment of the invention, high reflectivity DBRmirrors with alternating GaAs layer 730 and oxidized AlAs layer 720 canbe used in applications such as vertical cavity surface emitting laser(VCSEL) 700 shown in FIGS. 7a-7 c. FIG. 7a shows a top cross-sectionalview of VCSEL 700 with etched cavities 710 for allowing oxidation of thevarious buried high aluminum content layers and oxidized layer 740. FIG.7b shows a cross-sectional view along line 7 b of FIG. 7a while FIG. 7cshows a cross-sectional view along line 7 c of FIG. 7a. Upper DBR mirror799 is typically composed of pseudo-sinusoidally graded 25 layer pairs,typically with Al_(0.86)Ga_(0.14)As layers 798 alternating withAl_(0.16)Ga_(0.84)As layers 797. Upper DBR mirror 799 is silicon doped.Typically, the aluminum content of the AlGaAs layer with a high aluminumcontent should be sufficiently high to provide for a low refractiveindex while not being so high as to oxidize easily. The composition ofthe AlGaAs layer with a low aluminum content should typically havesufficient aluminum so that the low aluminum content layer isnon-absorptive at the lasing wavelength. Both Al_(0.86)Ga_(0.14)As layer798 and Al_(0.16)Ga_(0.84)As layer 797 are typically 0.15 wavelengthsthick and the graded AlGaAs material between Al_(0.86)Ga_(0.14)As layer798 and Al_(0.16)Ga_(0.84)As layer 797 have an aggregate thickness of0.2 wavelength resulting in a total optical thickness of ahalf-wavelength per layer pair. An Al_(0.16)Ga_(0.84)As currentspreading layer (not shown) and a p GaAs contact layer (not shown) aregrown above upper DBR mirror 799.

In accordance with an embodiment of the invention, lower DBR mirror 798is composed of 4 layer pairs, with GaAs layers 730 alternating withoxidized AlAs layers 720. AlAs layers 720 may contain a small amount ofGa for better structural integrity while still allowing rapid oxidation.Lower DBR mirror 798 is positioned over n-type GaAs substrate 777typically with an n-type GaAs buffer layer having a 200 nm thicknesssandwiched between lower DBR mirror 798 and GaAs substrate 777. For atypical wavelength of interest, λ=980 nm, the difference in refractiveindex leads to an interference effect that produces high reflection ifthe thickness of GaAs layers 730 is about 69.5 nm and the thickness ofoxidized AlAs layers 720 is about 163.3 nm. Since AlAs contracts byabout ten percent when oxidized, AlAs layers 720 have an initialthickness of about 179.6 nm.

Vertical cavity surface emitting laser (VCSEL) 700 has unoxidizedaperture 701 which is typically defined by oxidized Al_(0.94)Ga_(0.06)Aslayer 740 having a thickness of about a quarter wavelength. Activeregion 750 typically consists of a one wavelength thick separateconfinement heterostructure (SCH). The SCH has four InAlGaAs quantumwells (not shown) and five Al_(0.35)Ga_(0.42)As barrier layers (notshown). Transparent top electrode 780, typically indium tin oxide, andbottom electrode 790 provide electrical contacts. Current is injectedthrough epitaxially grown upper DBR mirror 799 passing through activeregion 750 and around lower DBR mirror 798 towards electrode 790. SinceDBR mirror 798 contains heavily oxidized aluminum layers, DBR mirror 798does not form part of the current path. Active region 750 is sandwichedbetween lower Al_(0.16)Ga_(0.84)As cladding layer 770 and upperAl_(0.58)Ga_(0.32)As cladding layer 760. The doping for lowerAl_(0.16)Ga_(0.84)As 770 is n-type with a doping level typically in therange of 1×10¹⁸/cm³ to 5×10¹⁸/cm³ and about a 100 nm thickness. Furtherdetails regarding VCSEL structure may be found in U.S. Pat. No.5,978,408 which is hereby incorporated by reference in its entirety.

A microlens may be formed by oxidizing a multilayer structure todifferent lateral extents. In accordance with the invention, FIG. 8shows microlens 801 fabricated using planar oxidation using etchedcavities 811. The different lateral oxidation extent of layers 801, 802,803, 804, 805, 806, 807, 809 and 810 in FIG. 8 is accomplished by havinga different aluminum composition in the respective layers.

Thin non-oxidizing layers 850 must be placed between oxidizing layers801, 802, 803, 804, 805, 806, 807, 809 and 810 to prevent the oxidationprocess from proceeding in the vertical direction. For example, ifmicrolens 801 has a radius of 25 μm with a focal length of 10 μm anembodiment in accordance with the invention is described in Table 1below. For the embodiment described in Table 1, the aluminum compositionin each layer is determined based upon the required oxide length at thesame oxidation time. Each layer thickness of Al_(x)Ga_(1−x)As is 1 μm.Based on the oxidation rate of AlGaAs as a function of aluminumcomposition at 400° C. the aluminum composition of each layer, thealuminum content of each layer 801, 802, 803, 804, 805, 806, 807, 809and 810 is selected to make the oxide front form a spherical surface. Ifa smoother oxide front is desired, layer thickness may be reduced andmore layers added. However, it should be noted that non-spherical lensesmay also be made using the lateral oxidation technique.

TABLE 1 Layer Oxidation length (μm) x (Al_(x)Ga_(1−x)As) 801 25 1 802 180.985 803 15.2 0.977 804 13.1 0.973 805 11.4 0.967 806 10 0.962 807 8.750.958 808 7.65 0.95 809 6.66 0.945 810 5.8 0.94

It is desirable to integrate various types of electronics and opticaldeviceson the same wafer. The etched cavity method provides an easymeans for forming electrical isolation regions between devices inoptoelectronics integrated circuits. FIG. 9 shows an embodiment inaccordance with the invention consisting of two GaAs metal-semiconductorfield effect transistors (MESFET) 905 and 911, integrated with edgeemitting semiconductor laser 1005. MESFET transistor 905 consists ofgate 931, drain 932 and source 933. MESFET transistor 911 consists ofgate 941, drain 942 and source 943. Signal is input on contact pad 950connecting to gate 941 and gate 931 along interconnect wires 991 and992, respectively. Source 943 is connected to contact pad 955 byinterconnect wire 993 with contact pad 955 going to ground. Interconnectwire 994 connects drain 942 to source 933. P-contact pad 965 isconnected to p-contact of edge emitting laser 1005 by interconnect wire995 and via 960 is connected to drain 932 by interconnect wire 996 andgoes to n-contact of edge emitting laser 1005.

Etched cavities 910 provide access to oxidize buried AlAs layer 1010(see FIG. 10). Buried AlAs layer 1010 functions to electrically isolateboth MESFET transistor 905 and MESFET transistor 911 from edge emittingsemiconductor laser 1005. To electrically isolate MESFET transistor 905from MESFET transistor 911 a shallow ion implant is used. A crosssection of FIG. 9 running across line 10 in FIG. 9 is shown in FIG. 10.The gate regions of MESFET transistors 905 and 911 are typically dopedto the low 10¹⁷/cm³. Higher doping levels at drain 932 and drain 942regions and source 933 and source 943 regions provide for better ohmiccontacts and can be accomplished by ion implantation. P⁺layer 1020,intrinsic layer 1030 and n⁺layer 1040 form the diode structure forsemiconductor edge emitting laser 1005.

For sufficiently short oxidation extents on the order of 5 μm, the shapeof the oxidation fronts follows that of the shape of the etchedcavities. FIG. 11a shows possible oxidation front shapes 1101, 1102,1103, 1104 and corresponding shaped cavities 1111, 1112, 1113, 1114,respectively. Cavities 1111-1114 are patterned, typically inphotoresist, and then etched. Subsequently, cavities 1111-1114 areexposed to water vapor in accordance with the invention to makeoxidation front shapes 1101-1104 in a buried AlGaAs layer. Oxidationfront shapes 1102, 1103 and 1104 concatenated together in FIG. to formoxidation front pattern 1199 which defines curved waveguide 1198.Oxidation front pattern 1199 is created by aligning cavities 1112-1114so that the oxidation front shapes 1102-1104 emanating from cavities1112-1114 meet to form a native oxide defined path which is curvedwaveguide 1198. Curved waveguides have applications to active devicessuch as ring lasers and integrated optical devices such as Mach-Zehndermodulators.

FIG. 13a shows distributed feedback (DFB) laser structure 1300 inaccordance with the invention where Al_(0.98)Ga_(0.02)As buried layer1320 is accessed through etched cavities 1321 for oxidation. Theperiodic wave-like shape of oxidation fronts 1325 shown in FIG. 13translates into a periodic refractive index variation. The periodicrefractive index variation when designed so that the periodicity is anintegral multiple of a half wavelength, provides a distributed feedbackmechanism for laser action. For example, for a refractive index of 3.5for AlGaAs buried layer 1320 which is typical for AlGaAs and awavelength of 820 nm for DFB laser 1300, the grating period for theoxidation fronts needs to be about 117 nm. FIG. 13b shows the layerstructure for DFB laser 1300. N type Al_(0.7)Ga_(0.3)As lower claddinglayer 1330 is grown on n-GaAs substrate 1329, typically to a thicknessof about 1 μm. N type Al_(0.4)Ga_(0.06)As waveguide layer 1331 is grownto a typical thickness of 0.12 μm above n type Al_(0.7)Ga_(0.3)Ascladding layer 1330. GaAs quantum well active layer 1335 lies between ntype Al_(0.4)Ga_(0.6)As waveguide layer 1331 and p typeAl_(0.4)Ga_(0.6)As waveguide layer 1332. P type Al_(0.7)Ga_(0.3)As uppercladding layer 1336 is separated from p type Al_(0.4)Ga_(0.6) Aswaveguide layer 1335 by oxidizable p-Al_(0.98)Ga_(0.02)As buried layer1320. P-GaAs layer 1340 serves as the capping layer.

FIGS. 14a-d show a typical fabrication sequence using etched cavities110 to access buried layer 120 of edge emitting laser structure 100 (seealso FIG. 1b) for oxidation in cross-section while FIGS. 15a-15 d show atop view. With reference to FIG. 14a and FIG. 15a, after growth of edgeemitting laser structure 100, SiO₂ layer 1405 is deposited on p typeAl_(0.5)In_(0.5)P cladding layer 155 by chemical vapor deposition (CVD),plasma enhanced CVD or evaporation. SiO₂ layer 1405 is followed bydeposition of photoresist layer 1401.

With reference to FIG. 14b and FIG. 15b, photoresist layer 1401 ispatterned into perforations 1410 which define the size and position ofetched cavities 110. Etched cavities 110 are etched through SiO₂ layer1405 by chemical or dry etching processes down to p typeAl_(0.5)In_(0.5)P cladding layer 155. P type Al_(0.5)In_(0.5)P claddinglayer 155 and underlying semiconductor layers are etched by dry etchingsuch as reactive ion etching (RIE) or chemically assisted ion beametching (CAIBE) typically down to GaInP etch stop layer 111. Forexample, etched cavities 110 in GaAs based edge emitting laser structure100 can be dry etched using chlorine-based reactive ion etching orchemically assisted ion beam etching.

In the case of AlGaAs alloys, the cavities can be etched using eitherdry or wet etching techniques. Details regarding layer oxidation may befound in H. Nickel, “A detailed experimental study of the wet oxidationkinetics of AlxGa1−xAs layers, J. Appl. Phys., vol. 78 (8), p. 5201,1995 and J. H. Kim et. al., ” Lateral wet oxidation of AlxGa1−xAs−GaAsdepending on its structures, Appl. Phys. Lett., vol. 69 (22), p. 3357,1996 which are hereby incorporated in their entirety.

An example of dry etching uses a Chemically Assisted Ion Beam Etcher(CAIBE) with a gas mixture consisting of 8 SCCM Cl₂, 2SCCM BCL₃ and 5SCCM Ar using a Technics Plasma RIBE ECR 3608. The gas mixture isactivated in electron cyclotron resonance (ECR) with 350 watts ofmicrowave power and 1200 gauss of magnetic field. The activated gas isaccelerated toward the sample at 550 V and neutralized by an electronemitter at 28% duty cycle. The sample is heated to 100° C. The etch ratedepends on cavity size and is about 1,000 Å per minute for a circularcavity with 2 μm diameter.

The cavity etch can also be performed using other dry etching tools suchas Reactive Ion Etching (RIE) using a PlasmaQuest ECR reactor. Anexample of RIE etching uses an ECR-activated mixture of 13.5 SCCM Ar, 1SCCM Cl₂, and 10 SCCM BCL₃ with a microwave power of 400 watts and RFpower of 33 watts. The etch chamber pressure is controlled to 2.1 torrand the sample temperature is set to 4° C. The etch rate also depends oncavity size and is about 730 Å per minute for 2 □m diameter circularpatterns and 1,300 Å per minute for cavities larger than 10 □m indiameter. The RIE machine is PlasmaQuest ECR reactor.

Although small cavities with high aspect ratios are difficult to makeusing wet etching, opening large cavities by wet chemical means is aviable option. A chemical mixture capable of etching AlGaAs is 1 H₃PO₄:1H2O₂:30 H₂O, where the numbers refer to volume ratios. Mixing thechemicals in other proportions is also possible. Moreover, otherchemicals such as HCl/H₂O₂ and H₂SO₄/H₂O₂ can be used.

Semiconductors other than AlGaAs will naturally require other etchchemistries. For Indium Phosphide, HCl diluted with H₃PO₄ works, whileH₃PO₄/H₂O₂ diluted with water can be used for InAlGaAs alloys. ForInGaAsP alloys, H₂SO₄/H₂O₂ diluted with water is ideal. CAIBE and RIEdry etching using chlorine-based gas mixtures like those mentioned forAlGaAs can also be used to open cavities in InP-based materials.However, the etch residues are less volatile, so the sample temperaturesshould normally be set higher than for AlGaAs etching. Etching processesfor materials other than AlGaAs and InGaAsP can be developed as needed.

With reference to FIG. 14c and FIG. 15c, photoresist layer 1401 isstripped and Al_(0.99)Ga_(0.01)As buried layer 120 is oxidized in themanner described in U.S. Pat. No. 5,262,360 which is incorporated byreference in its entirety. Edge emitting laser structure 100 is exposedto a saturated water vapor ambient at temperatures exceeding 400° C.Al_(0.99)Ga_(0.01)As buried layer 120 is converted into an amorphousaluminum oxide which also contains gallium and arsenic oxides.Al_(0.99)Ga_(0.01)As buried layer 120 is insulating and has a lowrefractive index in typically in the range of 1.5 to 1.6. Care should betaken prevent oxidation of stripe 170 of Al_(0.99)Ga_(0.01)As buriedlayer 120 which defines the waveguide core.

With reference to FIG. 14d and FIG. 15d, SiO₂ layer 1405 is removed bydry etching and p-electrode 105 is deposited by evaporation, with edgeemitting laser structure 100 tilted at about a 45 degree angle relativeto the beam direction of the evaporated metal atoms. Tilting edgeemitting laser structure 100 prevents metal from coating the interiorsurface of etched cavities 110 and creating undesired current pathsthrough edge emitting laser structure 100. Edge emitting laser structure100 is thinned and n-electrode 106 is deposited on the bottom to form ann-contact.

The same design methodology and fabrication process allows passivestraight waveguide structures 400 (see FIG. 4a) and 500 (see FIG. 5) tobe fabricated. However, passive straight waveguide structures 400 and500 have semiconductor layers in waveguide cores 425, 426, 427 and 526,527, respectively which do not absorb the propagating light. This meansthat SCH structure 190 (see FIG. 1b) is designed or processed so thatthe bandgap of SCH structure 190 in passive waveguide structures 400 and500 is larger than the photon energy of the light to be propagated. Thisbandgap engineering can be accomplished by, for example, impurityinduced layer disordering or migration enhanced epitaxy. The ability touse the same process for making passive waveguide structures 400 and 500as is used for active devices, such as edge emitting laser structure 100is important since this allows a single waveguide structure to haveactive and passive sections. The passive sections can be used as lightmodulators by applying a bias voltage which effects the absorptionproperties of the passive sections.

In summary, a method of accessing a buried oxidizable semiconductorlayer for lateral oxidation is described. Because wafer planarity ismaintained, the method benefits a wide variety of devices that utilizeburied oxidized materials for their optical and electrical properties.

As those skilled in the art will appreciate, other modifications,extensions, and changes to the foregoing disclosed embodiments of thepresent invention are contemplated to be within the scope and spirit ofthe invention as defined in the following claims.

What is claimed is:
 1. An optoelectronic integrated circuit comprising:a first active semiconductor device having a top surface and a bottomsurface; second active semiconductor device having a first surface; andan oxidized layer disposed between said bottom surface of said firstsemiconductor device and said first surface of said second semiconductordevice, said oxidized layer being exposed to an opening extending fromsaid top surface to said oxidized layer.
 2. The optoelectronicintegrated circuit of claim 1 wherein the first active semiconductordevice is a semiconductor laser and the second active semiconductordevice is a transistor.
 3. The optoelectronic integrated circuit ofclaim 2 wherein said first surface of said second active semiconductordevice is a top surface of said second active semiconductor device suchthat said oxidized layer separates said bottom surface of the firstactive semiconductor device from said top surface of said second activesemiconductor device.
 4. The optoelectronic integrated circuit of claim1 further comprising: a second oxidized layer, said second oxidizedlayer providing electrical isolation between said second activesemiconductor device and a third semiconductor device.
 5. Theoptoelectronic integrated circuit of claim 4 wherein the first activesemiconductor device is a semiconductor laser, the second activesemiconductor device is a first transistor and the third semiconductordevice is a second transistor.
 6. An intermediate structure for formingan optoelectronic integrated circuit comprising: a first activesemiconductor device having a top surface and a bottom surface; a secondactive semiconductor device having a first surface; and an oxidizedlayer disposed between said bottom surface of said first activesemiconductor device and said first surface of said second activesemiconductor device, said oxidized layer being exposed to an openingextending from said top surface to said oxidized layer, said openingfilled with an oxidizing agent.
 7. The intermediate structure of claim 6wherein the oxidizing agent is a moisture laden gas.
 8. The intermediatestructure of claim 7 wherein the gas is nitrogen.
 9. An integratedcircuit comprising: a first set of layers that include semiconductormaterial with which an optical device is formed; a second set of layersthat include semiconductor material with which an electronic device isformed; the optical device and the electronic device being electricallyconnected; and a buried layer between the first and second sets oflayers, the buried layer including material that is electricallyinsulating when oxidized; the integrated circuit further having one ormore etched cavities to access the buried layer for oxidation, materialin a region of the buried layer being sufficiently oxidized through theone or more etched cavities to electrically isolate the optical devicefrom the electronic device.
 10. The integrated circuit of claim 9 inwhich the optical device is a laser.
 11. The integrated circuit of claim9 in which the optical device and the electronic device are electricallyconnected through a via.
 12. The integrated circuit of claim 11 in whichthe electronic device includes first and second connected transistors,the first transistor being connected for receiving an input signal, thesecond transistor being connected to the optical device.
 13. Theintegrated circuit of claim 9 in which the oxidized region's shape andsize result from shape and placement of the one or more etched cavities.14. The integrated circuit of claim 9 in which the one or more etchedcavities include first and second etched cavities, an area of theintegrated circuit between the first and second etched cavities beingplanar.
 15. The integrated circuit of claim 9 in which the second set oflayers is over the buried layer, each etched cavity extending throughthe second set of layers to the buried layer.
 16. The integrated circuitof claim 9 in which the buried layer includes AlAs.